Method of making a ceramic article having open porous interior

ABSTRACT

An open porous ceramic article having an outer skin and an open porous interior. The interior has a continuous matrix of ceramic material which includes a plurality of chambers of predetermined shape and size distributed throughout the matrix. The chambers are in communication with one another; the passageways are smaller in cross-sectional area than the chambers connected thereby. With its skin removed, the article may be used as a filter for molten metal. With its skin intact, the article is useful as thermal insulation. 
     The open porous ceramic article may be made by coating a predetermined volume of thermally degradable beads with a predetermined quantity of binder capable of taking a set and thereafter molding the coated beads under pressure in a mold until the binder is set. A slurry of ceramic material fills the interstices of the mass of the coated beads. After the ceramic slurry has set, the resulting block is dried to remove the excess moisture from the ceramic material. Thereafter the dried block is heated to cause the beads to degrade and volatilize leaving a porous block of ceramic greenware which is thereafter further heated to complete manufacture of the article. The coated beads may be formed into a porous block in a mold prior to introduction of the ceramic slurry. Alternatively, the coated beads and the ceramic slurry may be mixed together before molding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a porous ceramic article, the pores of whichare in communication with one another. The article may be used as afilter, particularly for molten metals, for example, aluminum.

2. Description of the Prior Art

U.S. Pat. No. 3,939,079 discloses a filter medium which can be preparedby blending one hundred parts by weight of aggregate particles of atleast one of fused alumina, sintered alumina, silicon carbide, andsilicon nitride, 5-18 parts by weight of an inorganic bonding materialhaving a chemical composition of about 10 to about 50 percent by weightof SiO₂ and about 5 and about 20 percent by weight of B₂ O₃ andcontaining a major proportion of non-vitreous substance, and about 3 toabout 15 parts by weight of a combustible substance such as grainpowder, coke powder or an organic resin powder, kneading the mixturewith necessary amounts of a caking material and water, molding thekneaded mixture, drying the molded, kneaded mixture and thereaftercalcining the dried mixture at a temperature not lower than about 1100°C., during which time a number of air permeable pores of constant sizeare formed as a result of burning a combustible substance or theevolution of decomposition gases. The size and number of air permeablepores present in such filter medium are controlled by adjusting theamounts of bonding material and combustible substance, rather thanrelying on particle size of the aggregate as in the production ofconventional filter media using a vitreous bonding material.

U.S. Pat. No. 3,097,930 discloses a method of making a porous shape ofsintered refractory material by impregnating a sponge shape with asuspension or dispersion of refractory particles in a liquid followed byremoval of the liquid and material of the sponge by heating in an inertatmosphere to break down and volatilize the sponge material andthereafter further heating to sinter the refractory particles together.

U.S. Pat. No. 3,947,363 teaches that porous ceramic foam materialparticularly useful in filtering molten metal can be prepared byimpregnating an open cell flexible organic foam material with an aqueousslurry of ceramic material. The impregnated sponge is thereaftercompressed to expel a portion of the slurry while leaving the web coatedwith slurry. The impregnated sponge is thereafter released fromcompression so that the web remains coated and the material is dried.The dried material is then heated to first burn out the flexible organicfoam and then to sinter the ceramic coating thereby providing a fusedceramic foam having a plurality of intricate connected voids surroundedby a web of bonded or fused ceramic in the configuration of the flexiblefoam. This procedure is fully described in U.S. Pat. No. 3,893,917.

U.S. Pat. No. 4,024,212 describes the preparation of ceramic foams bythe impregnation of open-celled organic polymer foam material possessinga predetermined permeability and resilience with an aqueous slurry of athixotropic ceramic composition by passing the foam at least twicethrough preset rollers thereafter drying the resulting material andheating to remove the organic foam component.

U.S. Pat. No. 4,056,586 describes a method of preparing a molten metalfilter by impregnating an open cell hydrophilic flexible organic foammaterial with an aqueous ceramic slurry, thereafter compressing the foammaterial to expel a portion of the slurry while leaving the web-coatedtherewith, releasing the compression so the web remains coated withslurry, drying the coated material, and heating the dried material tofirst burn out the flexible organic foam and then sinter the ceramiccoating to provide a new ceramic foam having a plurality ofinterconnected voids surrounded by a web of fused ceramic in theconfiguration of the flexible foam.

U.S. Pat. No. 4,307,051 describes a process for manufacturing alight-weight refractory product by taking a particulate pore-formingmaterial or a mixture of refractory raw material with a particulatepore-forming material, providing the individual particles of thepore-forming material or agglomerates of particles or agglomerates ofthe mixture with a wet exterior, coating said wet exterior with dryrefractory raw material and firing the aggregate so produced. Expandedpolystrene in the form of hollow spheres is taught to be a desirableparticulate combustible material. The process described results in theproduction of a loosely sintered aggregate that is easily broken downinto individual hollow particles, the vast majority of which are in theform of small hollow spheres.

SUMMARY OF THE INVENTION

According to an aspect of the invention there is provided a method ofmaking a ceramic article having an open porous interior by coating apredetermined volume of thermally degradable beads with a predeterminedquantity of a binder that is capable of being set, thereafter placingthe coated beads in a mold, thereafter applying pressure to the coatedbeads in the mold and maintaining such pressure until the binder setsresulting in formation of a porous block, thereafter impregnating theporous block with a slurry of ceramic material, thereafter drying theimpregnated block, thereafter heating the dried impregnated block tocause the beads to degrade and volatilize leaving a block or porousceramic greenware, and thereafter further heating the block of ceramicgreenware to produce a ceramic article. The size of the pores isdetermined by the size or sizes of beads that are employed.

According to another aspect of the invention there is provided a methodof making a ceramic particle having an open porous interior whichincludes the steps of coating a predetermined volume of thermallydegradable beads with a predetermined quantity of a binder that iscapable of being set; mixing the precoated beads with an aqueous slurryof ceramic material; placing the resulting mixture in a mold andapplying pressure to the mixture thereby squeezing out the excessaqueous ceramic slurry and causing the coated beads to contact oneanother; maintaining the beads in contact with one another until thebinder is set to form a body; thereafter drying the body; thereafterheating the dried body to cause the organic beads to degrade andvolatilize leaving an open porous body of ceramic greenware andthereafter further heating the block of open porous ceramic greenware toform an open porous ceramic article. The size of the pores is determinedby the size or sizes of organic beads employed. The size of thepassageways which interconnect the pores is determined by the size ofthe beads as well as the amount of binder employed.

According to another aspect of the invention, there is provided a porousceramic article comprising a continuous matrix of ceramic materialincluding a plurality of chambers of a predetermined shape and sizedistributed throughout the matrix wherein said chambers are incommunication with one another via passageways whose area available forfluid flow is less than that of the chambers brought into communicationthereby.

DETAILED DESCRIPTION OF THE INVENTION

Open porous ceramic articles according to the present invention have ahighly porous interior due to their method of manufacture. Any thermallydegradable substance (by combustion or otherwise) whose coefficient ofthermal expansion is less than that of the ceramic material(particularly when in its green state) may be employed for the beads.Preferred are organic foams such as pre-expanded polystyrenes andpolyurethanes. It is especially preferred to utilize spheres of apre-expanded polystyrene foam beads as the void inducing member. Suchbeads are available in a variety of sizes. Those found to be mostsuitable for the manufacture of an open-cell ceramic foam materialsuitable for use as a molten metal filter range in diameter from about 1to about 6 millimeters. Preferrably in a manufacture of a given ceramicfoam material that is intended to be used as a molten metal filter theorganic beads are of substantially uniform spherical size and range indiameter from about 3 to about 6 millimeters. In other words, in anygiven article a narrow size distribution of spherical beads isdesirable. However, it is within the scope of the invention to employbeads of a material other than pre-expanded polystyrene foam. It is alsowithin the scope of the invention to employ particles of combustiblematerial that of a shape other than spherical, for example, cylindricalor spiral, or to use combinations of any of these sizes and shapes aswell as combinations of different compositions of combustible materials.

Preferred binders are those which are organic in nature and capable ofbeing broken down and volatilized on exposure to a sufficienttemperature less than that required to fully develop the desiredproperties of the finished ceramic matrix. A particularly preferredbinder is a chemically reactive epoxy resin. Preferrably the binder isthixotropic in nature to facilitate coating of the beads therewith, butto inhibit flow of the binder from the previously coated beads. Otherorganic binders such as Canadian balsam (pine sap) have been usedsuccessfully. The required properties of the binder include

(A) that the binder be capable of coating the beads uniformly with athin coating;

(B) that the binder be self-adherent so that two beads that have beencoated when brought into contact with one another will adhere to oneanother at their point of contact;

(C) that the binder be capable of taking a set such that after thecoated beads are brought into contact with one another and held in suchposition for a specified time under specified conditions that the beadswill become firmly attached to one another at their point of contact;

(D) that the binder not flow from the beads thereby completely fillingthe interstices between them;

(E) that the binder be capable of being decomposed and volatilized at atemperature less than that sufficient to sinter or fuse the ceramicmaterial of which the article is being formed; and

(F) that such decomposition and volatilization occur in the absence ofsufficient thermal expansion to cause rupture of the ceramic greenware.

Preferably, the ratio of predetermined volume of beads to predeterminedvolume of binder is from about 20/1 to 40/1.

Slurries suitable for manufacture of porous ceramic articles accordingto the present invention may comprise any suitable ceramic materialsuitable for the intended and use or service conditions for the openporous ceramic article. The slurry should be of low viscosity tofacilitate filling of the interstices of the porous block. The ceramicparticles of the slurry should be of small size, e.g. not larger thanabout 200 mesh so as to be able to flow into the interstices between thebeads. Use of too large a particle size may result in difficulty infilling the interstices of the porous block with the slurry andseparation of the slurry into fractions. It is desirable to include upto about two weight percent of a very fine silica-containing powder suchas that collected from the filtration system of calcination kilns. Thissilica powder imparts thixotropic properties to the slurry which helpsit flow into the interstices when pressure is applied to the system. Theshear forces present in the slurry (when it moves through theinterstices) allow the slurry to become fluid. As soon as the pressure(or shear inducing movement) is removed, the slurry thickens whichprevents it from draining out of the interstices. The slurry of ceramicmaterial should be capable of taking a chemically induced set. This canbe achieved by inclusion of a hydaulically setting cement such as acalcium aluminate cement or a chemical setting agent, e.g. phosphatebonded ceramics. Calcium aluminate cement hydrates upon contact with thewater in the aqueous ceramic slurry causing it to set into a solid afterbeing introduced into the porous network of beads. Cement is used toimpart strength unto the ceramic greenware body after the beads havebeen volatilized; otherwise, the structure will crumble.

Fluid property modifiers, such as Darvan^(TM), available from W. R.Grace and Company, Organic Chemicals Division, may be employed in theslurry to adjust its rheology. Darvan^(TM) is a polymer ion deflocculantwhich modifies the surface of the clay particles (Kaolin).

A suitable slurry of ceramic material is prepared by mixing together ona dry weight basis, the following ingredients: 2 percent fumed silicapowder, 10 percent calcium aluminate hydraulically setting cement, 3percent ZrO₂ flour, 2 percent kaolin, 23 percent calcined alumina, 60percent tabular alumina of 300-325 mesh particle size with an amount ofwater containing 0.67 percent by weight basis of Darvan^(TM) to form aslurry containing about 34 percent by weight of water. This ceramicslurry has acceptable fluid properties for use in the method ofmanufacture of an open porous ceramic article according to theinvention. This slurry may be forced into a previously formed porousblock as defined herein by application of hydraulic pressure to a moldcontaining the previously formed porous block of bonded beads.

It is not within the scope of this invention to optimize the propertiesof the ceramic slurry. The slurry compositions described herein areprovided to demonstrate operability of the invention. To provide bestperformance the properties of the desired type ceramic slurry should becharacterized and empirically optimized, e.g. particle sizedistribution, viscosity, pH and setting time. Once optimized, theseproperties should be monitored and controlled to assure reliable,repeatable results.

In one preferred method according to the invention, a ceramic articlehaving an open porous interior is made by taking a predetermined volumeof pre-expanded polystyrene foam beads and coating them with apredetermined quantity of binder that is capable of being set. After thebeads have been coated, they are placed in a mold and light pressure isapplied to the coated beads in the mold until the binder sets resultingin the formation of a bound porous block. Thereafter the porous block isimpregnated with a slurry of ceramic material. This may be accomplishedby introducing the slurry above the porous block and placing a moveablepiston or ram on top of the slurry and applying pressure to the pistonor ram to force the ceramic slurry into the porous block through theapplication of hydraulic pressure. A vacuum may be applied to the porousblock prior to or during introduction of the slurry to assistimpregnation of the porous block.

After impregnation the block is dried at a temperature below which thepre-expanded polystyrene foam beads would begin to degrade. This ispreferably accomplished by placing the impregnated block in an airconvection oven having an air temperature of between about 100° and 150°C. until the moisture level of the ceramic material is reduced to notmore than about eight percent and preferably lower. This may beconveniently accomplished by placing the impregnating porous block inthe drying oven over night.

After drying the impregnated block is heated to cause the polystyrene todegrade and volatilize leaving a block of porous ceramic greenware.During this heating step, the organic binder also decomposes andvolatilizes. This heating step may be conducted in an oxygen-containingatmosphere to facilitate removal and conversion of the organicmaterials.

After degradation and volatilization of the organic materials andparticularly the pre-expanded polystyrene beads, the block of porousceramic greenware is thereafter further heated to convert the ceramicgreenware into finished ware. This heating step causes the particles ofceramic material to sinter or fuse together to form a ceramic articlehaving a ceramic skin of low or no porosity and an open porous interiorwhich has a continuous matrix of ceramic material that includes aplurality of chambers of predetermined shape and size distributedthroughout the matrix. These chambers are derived from and correspond inshape to the beads although they are of somewhat smaller size than thebeads due to shrinkage which occurs upon sintering. Due to the method ofmanufacture, these chambers are in communication with one another viapassageways. When spherical particles are employed in making the porousblock, the area available for fluid flow of each of these passageways isless than that of the spherical chambers that are brought intocommunication thereby. The cross-sectional area available for fluid flowof the passageways can be increased by increasing the pressure appliedto the pre-expanded polystyrene foam beads during formation of theporous block of coated beads. Such pressure partially deforms the coatedbeads thereby increasing the amount of area of contact therebyincreasing the area of the resultant passageways. Conversely, when thepressure applied to the coated beads does not deform them, the contactarea is very low thereby resulting in very little passageway area. Theresulting finished ware is highly porous but is not highly permeable andis suitable as thermal insulation. A ceramic skin covers all surfaces ofthe exterior of the finished ware.

When it is desired to provide an open porous article, the skin isremoved, for example by sawing with a diamond-bladed bandsaw.

The following examples describe the preparation of ceramic articleshaving an open porous interior. According to the invention, all sieveanalysis figures are percentages of fractions using Tyler equivalentmesh. The packing densities and bulk densities are given in grams percubic centimeter (g/cc) and all fractions and ratios in thisspecification and the accompanying claims are given on a weight basisexcept where otherwise distinctly indicated.

EXAMPLE 1

A quantity of one millimeter diameter beads of pre-expanded polystyrenefoam were coated with Canadian balsam (pine sap) and placed in a moldand a slight amount of pressure applied. The pressure applied did notcrush the beads or deform them from a spherical shape. The mold wasformed of aluminum foil bound with masking tape. Thereafter a slurry wasmade containing 60 percent solids in water. On a dry basis the solidswere 20 percent by weight of CA-25 cement (calcium aluminatehydraulically setting cement available from Aluminum Company of America,Pittsburgh, Pa.) with the remaining 80 percent being calcined Al₂ O₃,minus 325 mesh. This ceramic slurry was poured into the matrix andallowed to hydrate for two days while exposed to air at ambienttemperature. Thereafter the impregnated block was dried in a circulatingair oven having an interior temperature of 100° C. Following the ovendrying step, the impregnated porous block was exposed to air at 400° C.to cause the pre-expanded polystyrene beads to be decomposed andvolatilized. Thereafter the porous ceramic matrix was fired at 1400° C.for 18 hours. The skin of the resulting ceramic article was thereafterremoved with a diamond-bladed band saw. The ceramic article cut veryeasily. The interior structure was very open porous and somewhatfriable.

EXAMPLE 2

A quantity of 1 millimeter diameter pre-expanded polystyrene foam beadswere bound together using a chemically setting epoxy system availablefrom ABATRON Incorporated of Gilberts, Illinois 60136. One hundred partsof epoxy resin ABATRON^(TM) 50-3 were combined with 13 parts ofABATRON^(TM) hardener 50-12. The beads were coated by placing them in abeaker and stirring them while pouring in the epoxy. Subsequently thecoated beads were placed in an aluminum foil mold and allowed to set fora full day. It is estimated that setting time was about 12 hours.Thereafter a slurry of the following composition was prepared: 2 percentbentonite, 16 percent CA-25 cement, 10 percent kaolin, 72 percentcalcined Al₂ O₃. The block of beads was positioned in a fixture so thata vacuum could be drawn through the porous block of beads duringintroduction of the slurry. Upon drying or setting of the slurry,cracking ocurred throughout the matrix. This is believed due to thebentonite swelling upon take up of the water from the slurry. Thisslurry did not impregnate the block readily because the slurry tended togel and become thick shortly after mixing.

EXAMPLE 3

A block was prepared from 1,000 milliliters of pre-expanded polystyrenebeads. One half of the beads were of small size (1-3 mm). The other halfof the beads were of larger size (5-6 mm). A total of 40 grams ofcatalyzed epoxy was utilized in coating the beads, 20 grams of epoxy toeach half. The small beads were first coated with the epoxy and placedinto the bottom of an aluminum foil mold. Thereafter the remaining 500ml of coated large beads were placed on top in the same aluminum foilmold. As in Example 2, the epoxy utilized was ABACAST^(TM) 50-3 resinand ABACURE^(TM) 50-12 hardener. Each portion of the beads was mixed byplacing them in a HOBART^(TM) mixer at a slow speed and adding thepreviously mixed catalyzed epoxy until the beads are initially coatedand thereafter adjusting the mixer to at or near to its highest speedfor about 2 minutes. The coated beads were allowed to remain in the moldover night while the epoxy set up. Thereafter the open porous bound beadblock was cut to desired shape on a band saw. The resultant porous boundbead block was impregnated with a slurry of the following composition:

    ______________________________________                                        CA-25 Calcium aluminate cement                                                                    150 grams                                                 kyanite             100 grams                                                 calcined Al.sub.2 O.sub.3                                                                         250 grams                                                 minus 325 mesh A-2 tabular alumina                                                                100 grams                                                 water               400 milliliters into                                                          which 3 cc of                                                                 Foamex ™ were added.                                   ______________________________________                                    

Foamex is a trademark for a mixture of aliphatic esters available fromGlyco Chemicals, Inc. It is employed to facilitate escapement of the airfrom the slurry as it impregnates the block. The porous bound bead blockwas readily impregnated with this mix. However, the mix apparentlycontained too much water and would not thicken or gel, and after removalof the block from the mold, part of the mix ran out of the impregnatedblock. Nevertheless, the impregnated block was fired in a kiln accordingto the following temperature-time schedule: ramp to 400° C. in two hoursfollowed by soaking at 400° C. for three hours followed by a ramp to1450° C. in 10 hours followed by soaking for 4 hours at 450° C. andthereafter a descending ramp to 20° C., i.e. ambient temperature. Theresulting ceramic article had an open porous interior. The resultingceramic article was cracked severely. This is believed due to the use ofkyanite, which has a 16-17 percent expansion when it converts to mulliteupon firing. The resulting ceramic article was very friable.

EXAMPLES 4-8

In each of the following examples, open porous blocks were made bytaking between 7 and 8 thousand milliliters of beads and stirring with150-250 gms of epoxy (Abatron^(TM) Epoxy Resin 50-3 and ABATRON^(TM)Hardener 50-12) in a large mixing bowl in a HOBART^(TM) mixer. A sheetof plastic film was placed over the mix bowl to prevent the beads fromoverflowing the top of the bowl during the stirring cycle. In some ofthe following examples the amount of epoxy resin/hardener was reduced toa total of 150 gms. per 7-8 liters of pre-expanded polystryrene beadsand this lesser amount was also found to be sufficient to bind the beadsto one another. After the beads were coated with the epoxyresin/hardener, they were placed in steel-lined wooden molds that hadbeen previously coated with grease or petrolatum. A spatula was used tospread out the beads as they were poured into a given mold. Afterfilling of the mold a wooden plunger was inserted into the top of themold and light pressure applied to compact the beads into contact withadjacent beads. Weights were placed on top of the plunger until theepoxy set, preferably overnight. The mold size was such that the openporous block dimensions were about 43/4 inches by 21/2 inches by 9inches, the 21/2 inch dimension being that of the thickness of the blockas set in the mold. After removal of the porous block from its formingmold, approximately 1/4 inch of its width was sliced off to obtain a41/2 inch by 21/2 inch by 9 inch rectangular block. The trimmed blockwas placed in a pressmold having dimensions closely approximating thatof the trimmed block except that the depth or height of the mold sidesexceeded that of the block (that is greater than about 21/2 inches). Theceramic slurry was poured on top of the porous block while in its pressmold and a plunger is placed onto the slurry and pressure applied to theplunger to force the ceramic slurry into the block of epoxy boundpolystyrene beads. Approximately 1000 gms to 2000 gms dry weight basisof ceramic slurry are needed to fully impregnate a block of suchdimensions. Before applying force to the plunger, note was taken as tohow far down the plunger should travel before contacting the styrofoamblock. It was not desired to compress the block; it was only desired toimpregnate the block with a ceramic slurry. Full impregnation orsaturation of the block with ceramic slurry was indicated by excessslurry being forced from around the plunger as the load on the plungerincreased. Following impregnation the porous block was removed from themold and placed under plastic film over night to permit full hydrationand partial drying of the ceramic material. The following day theimpregnated block was placed in a dryer at about 60° C. for 24 hours.Following this two-day hydration and drying cycle the dried block wasfired to provide a porous ceramic article. The rate of temperatureincrease during firing was maintained low enough to enable the beads todegrade and volatilize before the ceramic material was fired into finalform.

EXAMPLE 4

A ceramic slurry was formed which contained on a dry weight basis, 3percent fumed silica powder, 7 percent zirconia ZrO₂, 15 percent CA-25hydraulic cement, 58 percent tabular Al₂ O₃ (type A-303 tabular aluminaavailable from Aluminum Company of America minus 325 mesh) and 17percent A-2 tabular alumina (minus 325 mesh). To this was added 375milliliters of a water solution containing DARVAN in an amount of 3 ccper 500 milliliters of water. The resulting ceramic pressed very wellinto a porous block formed as described herein above. Upon firing,however, the product exhibited large cracks and much of it fell apart.

EXAMPLE 5

A porous block prepared as previously described was impregnated with aceramic slurry having the following composition on a dry weight basis;three percent fumed silica powder, 7 percent ZrO₂.SiO₂, 20 percentCA-25, 55 percent tabular Al₂ O₃ (type A-303 minus 325 mesh), 15 percentA-2 tabular alumina (minus 325 mesh). A slurry was made which on aweight basis contained 35.5 percent water. As in Example 4, the watercontained DARVAN^(TM) deflocculant. The porous block was readilyimpregnated with this slurry. Upon completion of the drying,decomposition and firing steps, the resulting ceramic article had anopen porous interior, exhibited very little cracking and had very goodstrength. The product appeared to have about 10 percent shrinkage on avolume basis and did not fall apart upon handling.

EXAMPLE 6

A slurry was prepared from the following ingredients on a dry weightbasis: 2 percent fumed silica powder, 10 percent CA-25 calcium aluminatecement, 3 percent ZrO₂ flour, 2 percent kaolin, 23 percent A-2 tabularalumina (minus 325 mesh), and 60 percent tabular Al₂ O₃ (type A-303minus 325 mesh). To a 1200 gram quantity of the ceramic ingredientsproportioned as just described were added 480 milliliters of water. Theresulting slurry contained 40 percent on a weight basis of water. Theresulting slurry was readily pressed into the porous block. After dryingthe impregnated block dimensions were 21/2 by 41/2 by 8-15/16 inches.After subsequent heating of the dried impregnated block to degeneratethe polystyrene beads and firing, the completed ceramic article had anopen porous interior and dimensions of 21/8 by 33/4 by 71/2 inches. Theresulting open porous ceramic article was of a nice appearance, wasquite hard and exhibited a very small amount of cracking on its bottomsurface. The resulting open porous ceramic article had a density of 0.64grams per cc (by calculation based on its weight and dimensions). Theresulting porous ceramic article contained about 83 percent by weight ofalumina.

Of the Examples 1-6 the product of Example No. 6 was rated the bestoverall.

Example 7

An open porous bound block of pre-expanded polystyrene beads wasprepared utilizing 6 parts by volume of large beads (4-6 mm spheres) and1 part by volume of small beads (1-3 mm spheres). To the 6 liters oflarge beads and 1 liter of small beads were added 170 grams of catalyzedepoxy. After the epoxy had set resulting block was impregnated with aceramic slurry of the following composition: 100 grams CA-25, 30 gramsZrO₂ flour (minus 325 mesh), 20 grams kaolin, 230 grams A-2 tabularalumina (minus 325 mesh), 600 grams tabular alumina (type A-303 minus325 mesh), and 30 grams of fumed silica powder. To the just describeddry ingredients were added 375 ml water, and a slurry was formed bystirring. The resulting slurry was readily pressed into the open porousblock of epoxy bound polystyrene beads. The ceramic article resultingafter firing exhibited about 80 percent porosity that was of an opennature within the skin.

EXAMPLE 8

A porous block of epoxy bound pre-expanded polystyrene foam beads wasprepared by taking 3 liters of beads (approximately 50 gms) and mixingthem with 52 grams epoxy/hardener. The coated beads were placed in amold to a depth of about four inches. The coated beads were compressedabout 1 inch from the point where initial die contact occurred (33percent compression based on original height). The load on the ramremained overnight while the epoxy set. Upon removal from the mold theblock looked very similar in appearance to a block prepared in theabsence of such substantial compression. The block was thereafter placedin a die and impregnated with a mix of the following composition: 30 gmsfumed silica powder, 100 gms CA-25 calcium aluminate cement, 30 gms ZrO₂flour (minus 325 mesh), 20 gms kaolin, 230 gms A-2 tabular alumina(minus 325 mesh), 600 gms tabular Al₂ O₃ type A-303 (minus 325 mesh). To500 gms mixture of the just described dry ingredients were added 200 mlof water to form a slurry. The resulting ceramic slurry was poured intothe block which had been previously placed in a die. The piston wasplaced over the slurry and pressed thereby causing the slurry toimpregnate the block. The die was thereafter inverted and the blockpushed to the opposite side of the die. Thereafter a second mix wasprepared utilizing the same dry ingredients in the same proportionspreviously described in this example. However, 250 ml of water wereadded to 500 gms of the dry ingredients. This second, thinner slurry waspoured onto the opposite of the open porous block in the die, theplunger inserted and loaded to force the second mix into the block. Theblock appeared to be completely impregnated at completion of thistechnique.

The foregoing description and embodiments are intended to illustrate theinvention without limiting it thereby. It will be apparent to oneskilled in the art that various modifications can be made in theinvention without departing from its spirit or scope.

What is claimed is:
 1. A method of making a ceramic article having anopen porous interior comprising the sequence of steps:(a) coating apredetermined volume of thermally degradable organic beads with apredetermined quantity of thermally degradable binder capable of beingset; (b) forming a slurry of ceramic material including a chemicallysetting cement; (c) mixing the coated beads and ceramic slurry togetherand placing the resulting mixture in a mold; (d) applying pressure tothe mixture in the mold to remove any excess slurry and causing thecoated beads to contact one another; (e) maintaining pressure on themixture in the mold until the binder sets and the cement sets resultingin formation of a body in which the beads contact one another; (f)drying the body; (g) heating the dried body thereby causing the beadsand binder to degrade and volatilize leaving ceramic greenware; andthereafter (h) further heating the ceramic greenware to form a porousceramic article comprising a continuous matrix of ceramic materialincluding a plurality of chambers of shape corresponding to that of thethermally degradable beads and of a size smaller than that of thethermally degradable beads distributed throughout the matrix whereinsaid chambers are in communication withone another via passageways whosearea available for fluid flow is less than that of the chambers broughtinto communication thereby.
 2. The method of claim 1 wherein the organicbeads are selected from the group consisting of pre-expanded polystyreneand polyurethane and range in diameter from about 1 mm to about 6 mm. 3.The method of claim 1 wherein the binder is an epoxy resin.
 4. Themethod of claim 1 wherein a surface of the mold is porous and has a poresize smaller than that of the organic beads.
 5. The method of claim 3wherein the body is dried by elevating its temperature and holding it atan elevated temperature less than that which causes chemical degradationof the organic beads until the moisture content of the body does notexceed eight percent by weight.
 6. The method of claim 1 wherein thedried body is heated to a temperature of between 150° and 400° C. andheld within such temperature range for a time sufficient to degrade andvolatilize the beads.